Comment 33 for bug 1070003

"Since my last post on the topic, GTK 3.4 brought in a lot of improvements, including among the others, support for most of the properties of the CSS3 backgrounds and borders family, linear gradients and the long requested inactive windows theming feature, which is showcased in GNOME 3.4"

previous posts links to this page: http://www.w3.org/TR/css3-background/

see there these:

3. Backgrounds
3.1. Layering Multiple Background Images
3.2. Base Color: the ‘background-color’ property
3.3. Image Sources: the ‘background-image’ property
3.4. Tiling Images: the ‘background-repeat’ property

 i don't know if this might be helpful or just spam .